This invention relates to golf balls, and, more particularly, to a golf ball having a core and a cover, either or both of which are formed by reaction injection molding fast curing thermoset polyurethane compounds.
Golf balls which are currently available fall into two general categories—balls which include a balata cover and balls which include a more durable, cut-resistant cover. Balata covers are made from natural balata, synthetic balata, or a blend of natural and synthetic balata.
Balata (trans-polyisoprene) was the primary material used for golf ball covers until the late 1960's. Balata covered golf balls were (and are) preferred by many golfers, primarily professional and low handicap golfers, because of the soft feel and because of the control which the better player can impart to the ball. However, balata covered golf balls are expensive, and difficult to produce. The process for producing a balata covered golf ball includes mixing of rubber, pre-forming half shells of the rubber compound, compression molding the half-shells around a wound golf ball core, post-curing the balata covers, and finishing the ball. The process is very labor intensive and expensive.
Conventional balata covered golf balls are produced by winding resilient, low heat-resistant elastic thread over a soft rubber or liquid rubber center, which must first be frozen to permit winding the rubber thread, and molding a conventional soft balata cover over the windings. The balata is conventionally vulcanized or crosslinked by using sulfur as the crosslinker or vulcanizing agent.
Most cut-resistant covers utilize Surlyn ionomer, which is an ionic copolymer of methacrylic acid and ethylene available form E. I du Pont de Nemours & Co. Other cut-resistant materials which can be used in golf ball covers are ionic copolymers available from Exxon under the name Iotek, which are similar to Surlyn ionic copolymers except that acrylic acid is used rather than methacrylic acid.
DuPont introduced ionomers in the late 1960's under the trade name Surlyn. Ionomers are copolymers of an olefin (generally ethylene), and an alpha-beta ethylenically unsaturated carboxylic acid (generally methacrylic acid). Neutralization of a number of the acid groups is effected with metal ions, currently chosen from the group of sodium, lithium, zinc and magnesium.
Ionomer covers are more durable than balata, and are currently the material of choice on most solid core, and many wound core, golf balls. Ionomers are thermoplastic materials, which allows (on sold cores) for the cover material to be injection molded around the core using a conventional injection molding process, that is, heating the thermoplastic material to above its melting point to a point where it will flow, and “injecting” the material in its molten state into a closed mold. The ionomer (thermoplastic material) cools and returns to its solid form, and can be removed from the mold easily. Thermoplastic materials are beneficial in that they can be heated, re-shaped, and cooled repeatedly, with no significant change in properties (as long as thermal decomposition does not occur). Ionomer covers, however, do not produce properties comparable to balata, and therefore are generally not considered acceptable for a premium golf ball.
Surlyn covered balls are cheaper than balata balls not only because Surlyn is cheaper than balata but because Surlyn balls can be processed after molding much easier and faster than balata balls.
The term “3 piece” is commonly used to refer to a golf ball which has a center, a layer of elastic windings over the center, and a cover. The term “2 piece” is used to refer to a golf ball which has a solid core and a cover.
Surlyn covered balls can be either 3 piece or 2 piece balls. Other variations include a solid core and two or more cover layers and a solid core, a mantle surrounding the core, and one or more cover layers over the mantle.
Other soft thermoplastic materials (such as thermoplastic polyurethanes, thermoplastic polyester elastomers, polyether block amide copolymers, etc.) have been used as covers on golf balls, with limited success. Although these materials, being thermoplastic, can be processed using conventional injection molding techniques as described above, these materials generally produce insufficient resilience properties to be used as covers for premium golf balls. U.S. Pat. Nos. 5,668,191, 5,759,676, 5,810,678 and 5,813,923 all illustrate the use of a thermoplastic polyurethane material as a mantle material. Thermoplastic polyurethane materials have also been used in blends with other materials, such as ionomers. U.S. Pat. No. 4,674,751 describes blends of thermoplastic polyurethanes with ionomers.
More recently, wound golf balls made using thermoset polyurethane materials for covers have been developed. Balls molded using thermoset polyurethane have many properties comparable to balata—similar feel (slightly harder) and comparable performance properties. Thermoset compositions suitable for use as cover materials for golf balls are described in U.S. Pat. Nos. 3,979,126, 3,989,568, 4,123,061 and 5,334,673. In all of the cited patents, a slow curing process is described. More specifically, in U.S. Pat. No. 3,979,126, the polyurethane cover requires a molding time of 45 minutes at 115° C., and 5 hours of curing time after demolding. In U.S. Pat. No. 3,989,568, half-shells are molded (for 5 minutes), and the polyurethane cover is compression molded around a core for 5.5 minutes, and required to post cure for 2 weeks at ambient temperatures. U.S. Pat. No. 4,123,061 (a continuation of U.S. Pat. No. 3,979,126), again describes a molding time of 45 minutes and a post-curing time of 5 hours at 100° C. U.S. Pat. No. 5,334,673 describes the need for an initial step of forming the urethane around the core, an intermediate curing step of 5 to 30 minutes which allows for the polyurethane cover material to retain the dimple form from the final molding step, and a final curing (molding) process of about 8 minutes. In all cases, the molding of thermoset polyurethane covers as defined in these patents is performed using compression molding processes.
There have also been a number of patents that describe a method for molding a thermoset polyurethane cover around a golf ball. U.S. Pat. No. 5,006,297 describes introducing a polyurethane into an open mold, allowing it to partially cure prior to introducing a core, introducing a core and molding the urethane around the core (an intermediate curing step), and then re-molding the urethane covered golf balls in mold cavities to form the dimple pattern. This process requires two separate molding operations and an overall process time of 2-plus hours. U.S. Pat. No. 5,733,428 describes introducing a thermoset polyurethane material into half-shells, closing a mold, and allowing the urethane material to cure for 10 to 15 minutes. This is performed in a multi-step operation, with material poured into each half of the mold in a separate operation, and the mold closed to “compression mold” the ball. U.S. Pat. No. 5,888,437 is a continuation of U.S. Pat. No. 5,733,428 and describes a process where the material and core are introduced into a mold, the mold is placed into a curing oven for 10 minutes, and then placed into a cooling chamber for another 10 minutes. U.S. Pat. No. 5,947,843 is also a continuation of U.S. Pat. No. 5,733,428, and describes a similar molding method to the other patents, that is, pouring urethane into open mold halves, then closing the mold, and curing the polyurethane. U.S. Pat. No. 6,132,324 describes the same process as U.S. Pat. No. 5,947,843, but pertaining to a multi-layer golf ball construction.
U.S. Pat. No. 6,083,119 describes a multi-layer golf ball. Column 13, lines 13–25 and column 43, lines 43–52 state that the inner and outer cover layers can be formed from materials which include thermoplastic or thermosetting polyurethanes/polyureas, including reaction injection moldable polyurethanes/polyureas. Column 19, lines 55–62 states that the inner and/or outer cover layer may comprise a thermoplastic polyurethane or a reaction-injection molded material such as one or more of the Bayflex RIM polyurethanes from Bayer. The patent does not describe any specific example of a golf ball which uses RIM polyurethane in either the inner cover layer or outer cover layer. The patent does not describe any process which molds RIM polyurethane.
Thermoplastic materials are generally processed using conventional injection molding techniques (heating a thermoplastic material under pressure until it reaches a molten state, injected into a closed mold, cooled until it returns to a solid state, and de-molded).
Thermoset materials used as covers for golf balls are all described as having either half-shells or material in its pre-cure form (urethane) introduced into an open mold, closing the mold, molding the cover under sufficient heat and pressure and allowing it to cure sufficiently to de-mold, cooling the mold, nd removing the molded part from the mold. In some cases, a post-curing step is required to complete the curing of the thermoset material.